"Rock of Ages" aspires to be the "Grease" of '80s hair metal. It ends up being "Mamma Mia!" with a dash of "Glee." The result is far from a joyous classic, but it's a perfectly fine backward-looking, trivia-centered guilty pleasure. Even if your reaction to Def Leppard and Poison is "Thank heavens Kurt Cobain came along," there are plenty of grins and giggles in this sweet-spirited rock 'n' roll fairy tale.

It could have begun with "Once upon a time in 1987." We open on a bus carrying shamelessly named ingénue Sherrie Christian (Julianne Hough) away from Nowheresville, Okla. Destination: the mixed-up world of pop-influenced life in L.A., where you can live the dream of your choice.

She has set her starry eyes on a singing career, and the thieves and streetwalkers who swarm when she steps onto the skeevy amusement park that is the Sunset Strip can't discourage her. Sherrie, a blissfully naïve Debbie Gibson type, is firmly grounded in fantasy.

She sees the world in terms of the jacked-up romanticism of Top 40 rock. In a snap (in this movie, everything happens in a snap) she's waitressing at a famous music club alongside fellow rock wannabe Drew (Diego Boneta).

Will they fall in love and ascend the slippery ladder of fame before the mayor's prudish wife (Catherine Zeta-Jones) and her church ladies shut down the place? Would we have a movie if they didn't?

The movie is chockablock with overqualified stars in extended, self-aware cameos, so many that it appears they were scooped up at a half-price sale. We get a daffy duet between rock bar owner Alec Baldwin and emcee Russell Brand, whose voices would stampede cattle. There's Paul Giamatti as a conniving manager, Mary J. Blige as a kindly gentleman's club owner and above all, Tom Cruise as an Axl Rose-style boozy sex god howler.

David James, Warner Bros.

Tom Cruise in "Rock of Ages."

Everybody gets a great turn or three before the cameras. They all propel the story in different directions, and it unravels. The embarrassment of actorly riches befuddles director/choreographer Adam ("Hairspray") Shankman, who seems unsure where to point the camera.

When it comes to rest on Hough and Boneta (a stage performer making his film debut), they look game but outgunned, and definitely outnumbered. Even the crowd scenes feature name extras: Look, over there, it's Kevin Cronin from REO Speedwagon and Skid Row's Sebastian Bach!

Not being a rock critic, I can't say much about Tom Cruise's singing. I will observe it's pretty funny to watch this well-preserved 50-year-old's ridiculous uber-masculine posturing as a love machine who would outshine Tom Jones in his prime. Wearing rock 'n' roll eyeliner, a Satan's-visage codpiece and S&M leathers, he seems to be having fun with his persona, striking a pose of open self-mockery. His performance says, "We both know this is ludicrous. Just relax and enjoy the show." Exactly the approach I recommend.